Why the cloud at night?

Why is there so much cloud at night recently when it’s been relatively clear through most of the day?  Well I have the AstroTrac ready and set up with the new Canon 5D MkII and the Canon 15mm fish-eye lens so that I can take some whole-sky pictures (the 5D and 15mm fish-eye gives me a full 180 degree field of view so I can get horizon-to-horizon shots).  So that’s why it’s cloudy 🙁  Don’t forget we are also in Perseid season (that’s the main reason I’m ready with the AstroTrac), and although we won’t be getting any Moon problems at the height of the shower, we can’t guarantee clear skies on the main nights, so it’s fingers-crossed time again, as usual.